Spell Burst
Instant
Buyback 
(You may pay an additional as you cast this spell. If you do, put this card into your hand as it resolves.)
Counter target spell with mana value X.
Bryan Sola
Rulings
For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.
(wotc • 2021-03-19)
Rulings
For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.
(wotc • 2021-03-19)